1. Field of the Invention
The present invention relates to a mass spectrometer and method of adjusting it.
2. Description of Related Art
A quadrupole mass spectrometer is an instrument which has a quadrupole mass filter generating a hyperbolic electric field and which operates to produce a selecting voltage by superimposing an RF voltage and a DC voltage on each other and to pass ions of only a desired mass-to-charge ratio by applying the selecting voltage and an axial voltage (that is a DC offset voltage applied to the four quadrupole electrodes equally). If only the RF voltage and axial voltage are applied to the quadrupole mass filter, this filter acts as an ion guide that passes all ions of a certain mass-to-charge ratio or higher. A triple quadrupole mass spectrometer is built by coupling two quadrupole mass filters and mounting a collision cell between them. Since the triple quadrupole mass spectrometer has two mass analyzers, this mass spectrometer provides higher ion selectivity than a single quadrupole mass spectrometer and thus is often used in quantitative and qualitative analysis.
In a triple quadrupole mass spectrometer, desired ions are first selected by the first mass analyzer. The ions selected by the first mass analyzer are normally known as precursor ions and guided into a collision cell including a multipole ion guide. An entrance electrode and an exit electrode are disposed on opposite sides of the ion guide. The ion guide has means (such as a needle valve) for introducing a gas from the outside. If a gas is introduced into the collision cell, precursor ions collide against the collision gas, producing fragmentation with a certain probability. As a result, the precursor ions are fragmented in the collision cell. These fragmented ions are known as product ions. Only intended ions of the precursor ions and the product ions in the collision cell are separated by the second mass analyzer and detected.
Sometimes, ion cooling is done on the upstream side of the first mass analyzer. In the cooling, ions are normally caused to collide with a gas by a multipole ion guide. The collision with the gas lowers the average kinetic energy of the ions and also reduces the range of kinetic energies. The cooling makes uniform the velocities of ions about to enter the first mass analyzer. This leads to improvements of resolution and sensitivity.
When quantitative analysis is performed, triple quadrupole mass spectrometers are often used in a mode known as multiple reaction monitoring (MRM). In this mode, ions of certain mass-to-charge ratios are selected by the first mass analyzer and the second mass analyzer, respectively. One measurement in which different ions are selected by the first and second mass analyzers is known as a transition. Transitions which are different in combination of selected ions are carried out in turn. Transitions performed in succession may interfere with each other. For example, if ions generated by a previous transition are left in the collision cell, and if the next transition is performed, then it is impossible to know which of the transitions produced the detected ions. In order to carry out multiple reaction monitoring (MRM) with reliable results, it is desired that ions in the collision cell be ejected as much as possible whenever the transition is varied.
JP-A-2010-127714 sets forth a method of accomplishing high sensitivity of the instrument by temporarily storing ions in a collision cell and then ejecting the ions. Since the ejected ions are pulsed, acceptance of noises is suppressed by accumulating pulsed portions as signals. In this way, the sensitivity of the instrument can be improved.
Even in this triple quadrupole mass spectrometer where ions are stored in the collision cell, it is important to eject ions from the collision cell as much as possible whenever the transition is varied. To reduce interference between transitions, the opening time in which ions are ejected should be increased. However, if the opening time of the collision cell is lengthened, high-speed multiple reaction monitoring can no longer be achieved. In this way, the opening time should not be too short or too long. The opening time needs to be optimized according to the tolerable range of interference between transitions.